Refactor helpers in Parser

[quephird]

Each parse helper called byparseStatement() should "know" the full grammar of the statement type it's attempting to parse. For example, parseForStatement() should actually be checking for the existence of the for token, not parseStatment(). That means all of the parse helps will need to return Statement? and the call site should have an if let to grab the return value, like so:

In parseStatement():

        if let forStmt = try parseForStatement() {
            return forStmt
        }
        .
        .
        . 

... then in parseForStatement():

        guard currentTokenMatchesAny(types: [.for]) else {
            return nil
        }
        .
        .
        . 

Look for other places in the parser where there is a similar splintering of knowledge.

[quephird]

Addressed in #23!